Lithography involves transferring a design or pattern onto a surface. In a typical lithographic process, a light source projects a design from a mask onto a surface. The surface is covered with a light-sensitive material called a resist film. Rather like photographic film, the resist film is exposed with the image from the mask. A thermo-chemical process, called post exposure bake, fixes the image in the resist film and various other processes transfer the image from the resist film to the surface.
Integrated circuits (IC) are often manufactured using lithography. The lithographic process can be repeated multiple times on an IC chip to build circuits in multiple layers. Market pressures continually drive toward smaller, more densely packed features in IC designs, leading to a number of design innovations such as optical proximity correction (OPC).
OPC refers to a variety of techniques that have been developed to compensate for optical proximity effects in lithography. That is, the critical dimension (CD) of a design is usually the smallest feature size in the design. For a CD near or below the wavelength of the light source, optical effects can have an impact on how the feature is projected onto the resist film. For instance, light diffracts at the edges of apertures in a mask. Small apertures create diffraction patterns that fan out from each aperture. The diffraction patterns from neighboring apertures can interact, potentially influencing the edge placement of projected features. In other words, a projected feature may appear larger or smaller depending on the relative proximity of other feature edges in a mask. This is often called an optical proximity effect, and the relative change in edge placement due to optical proximity effects is often called edge placement error.
OPC usually involves adjusting the edges of features in a mask layout to pre-compensate for edge placement error. That is, various models can be used to predict an edge placement for a given edge due to the proximity of other edges. Based on the prediction, the edge can be adjusted in the mask layout. For example, if the feature is predicted to appear too large, the feature's edge may be moved in slightly in the mask layout to reduce the size of the projection.